Attention has been directed to the problem of communicating between mobile stations on a continental land mass. It has been suggested that a satellite or satellites linked to ground stations could have coverage of the entire land mass for communication with mobile stations such as trucks and the like. To be useful, the satellite must be capable of communicating individually with a large number of different mobile stations scattered over a large land mass such as the continental United States. Mobile stations such as trucks cannot economically be fitted with a highly directive antenna for communicating with the satellite or with equipment for tracking the satellite with such an antenna nor can they carry sophisticated receivers or high-power transmitters. Instead, they carry relatively low gain dipole or monopole antennas, and consequently require a relatively high field strength for good reception, and can transmit only limited amounts of power toward the satellite.
Satellite transmitters are limited by the amount of power available from sources such as solar panels, and tend to be of relatively low power. At the present state of the art of commercial travelling-wave tube (TWT) amplifiers suitable for use in a satellite, they can produce about 100 watts of peak power at S band (about 2500 MHz). Consequently, very high gain is required at the satellite antenna in order to provide usable bit error rate (about 10.sup.-6 with a 10dB margin) at the mobile station.
The satellite is constrained as to the maximum weight which can be boosted into orbit. In addition, there is a tradeoff between the amount of fuel which can be carried by the satellite for station-keeping and the weight of large elements such as the antenna. A heavier antenna may limit the station-keeping fuel and thereby limit the useful life of the satellite. Consequently, it is very desirable to minimize the weight of the antenna. This, however, tends to limit its size, which conflicts with the requirement for high antenna gain. There may also be other size limitations on the satellite antenna, due to the size of the launch vehicle. For example, the ARIANE launch vehicle has a diameter of about 3.5 meters. Thus, a 3.5 meter diameter reflector can fit within the envelope, but any larger reflector would have to be furlable. Furlable reflectors are undesirable for cost and reliability purposes.
The relatively small size of a 3.5 meter reflector limits the gain of the satellite antenna. For radiated antenna beams directed to separate portions of the continental United States, with each beam being produced by one feed horn located in the focal region of the reflector, the mobile station might have satisfactory reception if it happened to be located at the beam center, but the beam shape would be such that at the transition between beams directed toward adjacent portions of the land mass, the signal level might well be to low for satisfactory reception. Beams produced by one feed horn for each separate beam directed to a region of a land mass tend to overlap at about a -4 or -5 dB level, as illustrated in FIG. 4. In FIG. 4, a reflector 400 is illustrated as being above the continental United States, illustrated by outline 410. A first single feed horn 412 located in the focal region of reflector 400 is illustrated as feeding reflector 400 to produce a beam centered on a point 414 having a 41/2 dB contour illustrated as a dotted line 416. Another single feed horn illustrated as 420 also located in the focal region generates a beam displaced from the beam produced by horn 412. The second beam has a center at
point 422 on the land mass and a 41/2 dB contour illustrated as 424. While feed horns 412 and 420 are illustrated as separated, they are actually closely spaced or contiguous, and cannot be brought close enough to cause beam overlap at, say, the 1 dB contour. Thus, a vehicle moving from point 414 to point 422 would experience a 4 to 5 dB loss of received signal power at the beam transition, which might have a very adverse effect on the bit error rate. While only two beams are illustrated in FIG. 4, the satellite may produce four or five or more as required for the desired coverage.
With the expected antenna gains of the satellite antenna, and for a dipole or monopole antenna at the mobile station, 50 watts peak power at the satellite could provide a single user with a bit error rate of about 10 .sup.-6 with a 10db margin at the beam peak (points 414 or 422 of FIGURE 4). Concurrent use of the channel by other users would not be possible because of frequency overlap and power limitations. If frequency division multiplex were used, truck drivers and other nontechnical personnel might be required to select the proper frequency for use, depending upon where they happened to be on the land mass, which might result in confusion. Time-division multiplex could be used, but transmission delays could be expected and extensive synchronizing techniques would be required.